Understanding Legume Benefit in No-till Crop Rotations through Microbial Activity Analysis.

In order to better understand legume benefit to corn and wheat performance in crop rotations, a two-year study was carried out in 2006 within an established no-till crop rotation experiment in Brookings, SD. Two crop phases (corn and winter wheat) were chosen in four different rotation treatments (field pea/winter wheat/soybean/corn; soybean/spring wheat/field pea/corn; soybean/corn/field pea/winter wheat; soybean/ corn/oats/winter wheat). Corn preceded by soybeans and fieldpeas and winter wheat preceded by oats and field pea were sampled for the analysis. Crop residue of preceding crop and subsequent corn and winter wheat grain were sampled and analyzed for yield, nutrient content and fiber content. Microbial mineralization of endogenous soil carbon and added substrates assessed using the BDoxy 96-well plate method prior to winter wheat and corn planting. Twelve different carbon substrates (including “none”) with or without nitrogen (N) and phosphorus (P) addition were added to the soil slurry inoculum from the crop plot soil and mineralization activities were measured. Corn and wheat grain yield and quality were not significantly different in both years. Soybeans yielded significantly (P < 0.05) more residue biomass and contained more lignin than fieldpeas. Microbial mineralization was significantly (P < 0.05) higher in soybean-harvested soil than in fieldpeas-harvested soil when different substrates and nutrients were added. Microbial mineralization in fieldpeas and oat-harvested soil was not significantly different but the addition of N significantly increased activity. Fieldpeas root extract was a preferred substrate by the microbes. From these studies, microbial activity analysis helped explain preceding crop contribution in the rotation. It is also shown that legume contribution differs with legume specie.